Along the border between Norway and Finland lies the world’s greatest Atlantic salmon river. To the Norwegians, it is the Tanaelva or Tana; the Finns call it the Tenojiko or Teno. But to both countries it is known as one of the purest and cleanest rivers, passing through largely unspoiled and unpolluted regions from Finnmark, Norway’s northernmost county, into Lapland. Now, however, the river’s waters and banks are crowded with rotting fish, their skin peeling away, and the air is thick with the odour of decaying flesh. “It’s a mess. It’s quite dramatic, says Aino Erkinaro, a doctoral researcher from...